1. Field of the Invention
The present invention relates to a stator core which is used in a rotary electric machine such as an electric motor or the like. It also relates to a method of assembling the stator core as well as to that apparatus for assembling the stator core which is used in carrying out the method.
2. Description of the Related Art
As can be seen in the Japanese Published Unexamined Patent Application No. 124241/1986, there is conventionally known the following stator core. Namely, a segmented (or divided) core is made up of an arcuate yoke portion, a pole portion which extends diametrically inwards from the yoke portion, and a tooth portion on a diametrically inner end of the pole portion. On one circumferential end and on the other circumferential end of the yoke potion, there are formed a projection and a concave groove (hereinafter simply called a groove), respectively. A plurality of segmented cores each being formed as described above are circumferentially connected together (i.e., connected together in the circumferential direction) such that a projection of one of adjoining segmented cores is fit into a groove of the other of the adjoining segmented cores, thereby assembling a stator core. other of the adjoining segmented cores, thereby assembling a stator core.
In the above-described conventional stator core, the groove is formed into a dovetail groove and the projection which is formed into a dovetail shape is fit by pressing into the groove in an axial direction of the stator core.
Since the stator core is relatively large in its axial length, in the above-described conventional one in which the projection is fit into the groove in the axial direction, the stroke of press fitting becomes long. In order to prevent the load of press fitting from becoming excessive, the interference between the projection and the groove must be made small. As a result, the segmented cores are likely to give rise excessive play or are likely to cause rattling among them.
The present invention has a first object of providing a stator core in which this kind of disadvantage has been eliminated.
Further, in the above-described conventional stator core, the work of subjecting the segmented cores to press fitting in the axial direction must be carried out in sequence one at a time. It has therefore a disadvantage in that the stator core takes much time to assemble.
In view of the above-described disadvantage, the present invention has a second object of providing a method of assembling the stator core with a good working efficiency as well as of providing an apparatus for assembling a stator core which is used in carrying out this method.
In order to attain the above-described first object, the present invention is a stator core having a plurality of circumferentially segmented cores, each of the segmented cores having a projection and a groove on circumferential one end and the other end, respectively, of a yoke portion thereof. The segmented cores are assembled to form a stator core by circumferentially connecting the plurality of segmented cores by fitting a projection in one of adjoining segmented cores into a groove in the other of the adjoining segmented cores. The stator core is characterized in that: a straight portion is formed in an intermediate portion of each of the projection and the groove, the straight portion extending straight in a predetermined width in a normal direction which is normal to each end surface of the yoke portion; that a front end portion of the projection and an inner bottom portion of the groove are formed into a semicircular shape having a diameter equal to a width of the straight portion; and that a contracted portion which adjoins the straight portion and has a width smaller than the width of the straight portion, and an enlarged width portion which extends in width from the contracted portion towards each end surface of the yoke portion are formed in those portions of the projection and the groove which are respectively closer to each end surface of the yoke portion; such that, when the projection is fit by press fitting into the groove in the normal direction, the contracted portion of the groove is enlarged in width within an elastic region of groove walls on diametrically outer side and inner side, respectively, of the groove to thereby allow the straight portion of the projection to pass therethrough and, that once the straight portion of the projection has passed through the contracted portion of the groove, the straight portion and the contracted portion of the groove are brought into forced contact with the straight portion and the contracted portion of the projection, respectively.
According to the above-described arrangement, it becomes possible to fit by press fitting the projection of one of circumferentially adjoining segmented cores into the groove of the other of the adjoining segmented cores in the normal direction which is normal to the end surface of the yoke portion, i.e., in the circumferential direction. Therefore, the stroke of the press fitting becomes extremely shorter than the one in which the projection is fit by press fitting in the axial direction. As a result, even if a relatively large interference between the projection and the groove is maintained, the load of press fitting will not become excessive. Further, due to the presence of the straight portions, the tilting of the segmented cores with the fit portion of the projection and the groove functioning as a fulcrum can be prevented. In conjunction with the fact that the interference can be sufficiently secured, the rattling of the segmented cores can surely be prevented. Therefore, a stator core of high quality can be obtained.
If the groove is formed such that the widthwise center line of the straight portion coincides with the normal line that passes through the widthwise center of the end surface of the other end of the yoke portion, the groove wall on the diametrically outer side of the groove becomes thinner than the groove wall on the diametrically inner side of the groove. As a result, the bending rigidity of the groove wall on the diametrically outer side of the groove becomes smaller than the bending rigidity of the groove wall on the diametrically inner side of the groove. Therefore, when the projection is fit by press fitting into the groove, only the groove wall on the diametrically outer side is deflected, and there will occur a step (or a stepped surface) in the diametrical direction between the adjoining segmented cores.
Therefore, it is preferable to form the groove in a manner that a widthwise center line of the straight portion is offset diametrically inwards relative to the normal line passing through the widthwise center of the end surface of the other end of the yoke portion such that the bending rigidity of the groove wall on diametrically outer side of the groove becomes equal to the bending rigidity of the groove wall on diametrically inner side thereof.
Further, if the length of the straight portion of the projection is made slightly longer than the length of the straight portion of the groove to secure an interference in the normal direction between the contracted portions of the projection and the groove, the play among the segmented cores in the circumferential direction can advantageously be removed completely.
In order to attain the above-described second object, the present invention is a method of assembling a stator core which has a plurality of segmented cores. Each of the segmented cores has an arcuate yoke portion, a pole portion extending diametrically inwards from the yoke portion, and a tooth portion on a diametrically inner end of the pole portion. The yoke portion has a projection and a groove on circumferential one end and the other end, respectively, thereof. The segmented cores are assembled to form a stator core by circumferentially connecting the segmented cores by fitting a projection in one of adjoining segmented cores into a groove in the other of the adjoining segmented cores. The method comprises the steps of: arranging the projection circumferentially insertable into the groove; disposing the plurality of segmented cores into an annular shape; and then pushing the plurality of segmented cores radially inwards in a manner synchronized with each other; such that the projection in each of the segmented cores is fit by press fitting into each of the grooves in the adjoining segmented cores in a circumferential direction.
Further, an apparatus for carrying out the above-described method comprises: a bed; a movable plate which is provided above the bed so as to be movable up and down; a plurality of jigs which hold the segmented cores and are radially disposed on the bed so as to be radially movable; and cam means which is disposed under the movable plate so as to push the plurality of jigs radially inwards simultaneously with a downward movement of the movable plate.
In this case, preferably each of the jigs has a recessed potion into which each of the segmented cores is inserted in position from an upper side, and an engaging groove with which the tooth portion of each of the segmented cores is engaged for positioning in the circumferential direction.
When the movable plate is lowered in a condition in which each of the segmented cores is set in position on each of the jigs, the plurality of jigs are pushed radially inwards in a manner synchronized with each other. The plurality of segmented cores that are held by these jigs approach each other in the circumferential direction, whereby the projection of each of the segmented cores is fit by press fitting into the groove of each of the adjoining segmented cores in the circumferential direction. In this manner, the stator core can be assembled efficiently by a single operation, resulting in a largely improved workability.
Further, if the tooth portion of each of the segmented cores if set in position by forming the engaging groove in each of the jigs as described above, the circumferential clearance between the tooth portions of the respective segmented cores can be accurately controlled. Therefore, the occurrence of cogging (i.e., an irregularly aligned surface like in a cogged joint) due to the dispersion in the clearances can be prevented.